This invention relates to novel copolyimides in general and more particularly to novel copolyimides derived from the reaction of one or more aromatic dianhydrides with a meta-substituted phenylene diamine and a bridged aromatic diamine. The incorporation of the meta-substituted phenylene diamine derived units and bridged aromatic diamine derived units into the linear aromatic polymer backbone results in a copolyimide of improved flexibility, processability and melt-flow characteristics.
Aromatic polyimides are generally difficult to process because they exhibit only a limited degree of flow even at high temperatures and when subjected to high pressure. These materials are, however, exceptionally thermally stable and resist attack by most solvents. Additionally, they generally have very high glass transition temperatures because of their aromatic character. Because of desirable properties such as resistance to solvents and the high glass transition temperature, many attempts have been made to prepare aromatic polyimides which can be easily processed.
Prior art attempts such a solution by incorporation sulfur linkages into a polyimide backbone. Another teaching is the use of particulated oligomeric polyetheramide acids which may be converted to a high molecular weight polymer system by melt polymerization. Another discloses the use of varied proportions of different polyetherimide segments in an attempt to reach the optimum balance between processability and solvent resistance. Another teaching is that certain polyetherimides can be prepared which can be reinforced with various fillers to form composites. Prior art further teaches that the incorporation of flexible moieties into the backbone of a polyimide can increase thermoplastic character, and that the incorporation of phenylene ether units into polyimides can improve melt-flow properties. Even though all of the above prior art systems have contributed in various ways to improving the processability of linear aromatic polyimides, there is still a definite need in the art for enhanced melt-flow properties in polyimides in order that they may be used for applications such as hot-melt adhesives or as matrix resins for fiber-reinforced composites.
Accordingly, an object of the present invention is to provide polyimides with improved flow properties.
Another object of the invention is to provide polyimides with high glass transition temperatures and which soften to a high degree above their glass transition temperatures.
Yet another object of the invention is to provide polyimides with resistance to solvents.
A further object of the invention is to provide polyimides which can be solvent or hot-melt coated onto fibers for preparation of composites.
Another object of the invention is to provide polyimides which can be used as hot-melt adhesives.
Still another object of the invention is to provide polyimides which have low melt viscosities.
Another object of the invention is to provide polyimides which become more planar when exposed to temperatures above their glass transition temperature.
Still another object of the invention is to provide a process for making polyimides of enhanced melt-flow characteristics.
A further object of the invention is to provide a process for using the polyimides of the invention as hot-melt adhesives for bonding materials together by surface attachment.